In the case of such LED arrays, on account of the steep U/I characteristic curve of LEDs, even small changes in the forward voltage can bring about a great change in current and thus lead to a considerable deviation of the current intensity in the individual LED chains of the LED array from a predetermined desired current intensity.
A variation of the forward voltage of LEDs may, on the one hand, be dictated by production. A fine grouping of the LEDs with regard to the forward voltage (i.e., for each group the range for the forward voltage is comparatively small, so that the number of groups is guite high) is conceivable in order to solve the problem outlined above. This is associated with comparatively high costs since corresponding logistics and stockkeeping are necessary.
On the other hand, the forward voltage of an LED is temperature-dependent, and it is possible for various temperature dependencies to occur, in turn, between individual LEDs. Therefore, a change in temperature may lead to a change in the forward voltages. In order to counteract an associated change in the current intensity in the LED chains, an electrical resistor is connected in series with each LED chain, for example, in the case of conventional circuits. Said resistor leads overall to a flatter U/I characteristic curve of the relevant LED chain, thereby achieving a certain limitation of the current in the LED chain. However, rising accuracy requirements when complying with a predetermined current distribution between the individual LED chains are accompanied by an increase in the magnitude of said resistor and thus the voltage dropped across the latter, thereby impairing the efficiency of the overall system.
Furthermore, an alteration of the forward voltage of an LED chain may also be caused by the failure of individual LEDs, for example due to a short circuit of an LED. In the case of a current setting by means of series-connected resistors, this leads to a major redistribution of currents in the LED chains.